Add `smtp_password_file` configuration option

[eritho23]

I want to use a secret manager when deploying Radicale. The secret manager (sops-nix) puts secrets in /run/secrets/*, so an option for setting the SMTP password path would be helpful, in order for us to be able to use the SMTP hook.

I am willing to work on this issue when I have time.

[pbiering]

can make sense, looks like an additional option would be then required like smtp_password_source with default config://smtp_password and optional file:///path-to-file.

Feel free to submit a tested PR for that.